Topoisomerase I inhibitors have shown positive effects in combination with radiation therapy in some studies. Normally oxygenated and hypoxic human MCF-7 breast carcinoma-cells were exposed to irinotecan (100 mu M or 250 mu M) or to SN-38 (10 mu M or 25 mu M) for 1 h prior to, during and for 3 h after radiation. Irinotecan and SN-38 showed little or no radiation sensitization of normally oxygenated MCF-7 cells but were effective radiation sensitizers of hypoxic cells. Both irinotecan and SN-38 diminished or eliminated the shoulder of the radiation survival curves of both the normally oxygenated and hypoxic cells indicating inhibition of the repair of sublethal radiation damage to DNA. Irinotecan (20 mg/kg or 30 mg/kg) was administered to mice bearing the EMT-6 mammary carcinoma on days 7 through 11 just prior to fractionated radiation (5x3 Gray). The tumor growth delays obtained with the combination regimens were greater than expected for simple additivity of the two treatments. Treatment with irinotecan resulted in decreased expression of topoisomerase I mRNA and increased expression of topoisomerase II mRNA in EMT-6 tumor tissue. Irinotecan treatment did not alter the protein levels for topoisomerase I or II in the tumor tissue; however, the combination of radiation therapy and irinotecan administration resulted in decreased topisomerase I and increased topoisomerase II protein in the tumor tissue. These results suggest that with appropriate scheduling of a topoisomerase I inhibitor and a topoisomerase II inhibitor with fractionated radiation therapy maximal cyto-reduction can be achieved.

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